Unix socket authentication plugin should support logging in as a different user

MySQL auth_socket plugin supports logging in as a different user:
alter user backup identified with auth_socket as 'root';

This way, the backup script could run as root, and the permissions the backup tool has is the permissions of the 'backup' user. This avoids storing a backup user password into a configuration file, improving security.

Please implement the MariaDB unixsocket authentication plugin to supprort logging in as a different user as well.